The invention relates to a battery system and a method for providing an intermediate voltage, in particular in traction batteries for electric vehicles.
Electric energy sources, in particular mobile electrical energy sources, for example traction batteries for electrically operated vehicles, frequently comprise rechargeable energy stores, for example lithium-ion batteries, which energy stores provide an output voltage of more than 12 volts. Control devices are used to control and monitor the energy store, which control devices typically require a supply voltage, for example exactly 12 volts, which is lower than the high voltage that is provided by the energy store. This low voltage or also vehicle voltage can be provided by a second battery, for example a lead acid battery or generated by means of a DC current conversion from the high voltage of the energy store.
Energy stores for electric vehicles frequently comprise a series connection of where necessary parallel connected individual cells that comprise a low intrinsic voltage. A total voltage can be generated from the low intrinsic voltages by way of the series connection of the individual cells, which total voltage can be used in a range that spans several hundred volts for providing the high voltage, for example the traction voltage.
Owing to the fact that individual cells of an energy store comprise production related variations of the performance parameters, the individual battery cells are subject to greatly different loadings and/or comprise different charge conditions after a particular period of operation of the energy store despite having the same starting charge condition. Owing to the fact that battery cells with the weakest charge condition significantly influence the performance of an energy store that is constructed in a series connection, methods are used that can ensure a so called “cell balancing”, in other words a balancing of the charge conditions of the individual battery cells. In particular, in the case of lithium-ion batteries, the performance, serviceable life and efficiency of the entire energy store can be improved using these methods. It is possible using active methods to move a capacitive or inductive charge from battery cells that have a higher charged condition to battery cells having weaker charge conditions using active methods. Passive methods partially discharge selective individual battery cells, in dependence upon their relative charge condition.
A method for cell condition balancing in a battery system having series-connected battery cells is disclosed in the publication WO 2007/128876 A1, in which weaker battery cells are selectively supplied with energy from cell-specific charge devices in order to equalize the charge condition of all the battery cells.